Letter pubs.acs.org/JPCL
Effects of Exciton Polarity in Charge-Transfer Polymer/PCBM Bulk Heterojunction Films Brian S. Rolczynski,†,‡,§ Jodi M. Szarko,†,‡,§,⊗ Hae Jung Son,‡,∥,⊥ Luping Yu,*,‡,∥ and Lin X. Chen*,†,‡,§ †
Department of Chemistry and ‡ANSER Center, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States § Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States ∥ Department of Chemistry and The James Franck Institute, The University of Chicago, 929 East 57th Street, Chicago, Illinois 60637, United States S Supporting Information *
ABSTRACT: Charge-transfer copolymers with local electron density gradients, systematically modified by quantity and position of fluorination, result in widely variable (2−8%) power conversion efficiencies (PCEs). Ultrafast, near-infrared, transient absorption spectroscopy on the corresponding films reveals the influence of exciton polarity on ultrafast populations and decay dynamics for the charge-separated and charge-transfer states as well as their strong correlation to device PCEs. By using an excitation energy-dependent, dynamic red shift in the transient absorption signal for the polymer cation, the exciton polarity induced by push−pull interactions within each polymer fragment is shown to enhance charge dissociation on time scales of tens to hundreds of picoseconds after excitation. These results suggest the important role played by the local electronic structure not only for exciton dissociation but also for device performance. SECTION: Energy Conversion and Storage; Energy and Charge Transport
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of these CTPs, the poly(thienothiophene-benzodithiophene) (PTB) series, has established new PCE records previously set by P3HT. Understanding why these CTPs could improve device PCEs at the fundamental steps of OPV function is important for establishing design guidance to modulate their properties via chemical modification because both their molecular properties and corresponding device PCEs are highly sensitive to small differences in local molecular structure.10−12 A clear correlation has previously been observed between the charge-carrier population and the driving force for exciton dissociation in BHJ OPV films containing homopolymers or CTPs as D and fullerene derivatives such as phenyl-C61-butyric acid methyl ester (PCBM) as A.9,13 An anomalous deviation was found in certain CTPs by Durrant and coworkers, which has higher yields of charge-carrier population despite a much smaller apparent LUMO−LUMO energy offset, which is an estimate of the driving force for electron transfer from the exciton of the polymer to PCBM.9,13 The driving force can also be approximated by comparing the ionization potential and electron affinity of the donor and acceptor, respectively. This unexpected result prompts reconsideration of the driving force
rganic photovoltaic (OPV) devices have great potential for renewable, clean, cost-effective energy production.1−3 However, one of the main challenges is their relatively low power conversion efficiency (PCE) with respect to their inorganic counterparts.4 In the past several years, the PCEs of OPV devices have almost doubled from 5% to nearly 10% in single junction devices.5 One of the key factors responsible for such significant improvements in PCEs is the incorporation of charge-transfer polymers (CTPs). In contrast with homopolymers such as poly(3-hexylthoiphene) (P3HT), which contain identical blocks along the polymer backbone, CTPs are composed of distinct building blocks with different electron affinities in each repeating unit of the polymer. As a result, they form (d-a)n sequences, where n is the number of the repeating units of the polymer and d and a are adjacent blocks with some degree of donor−acceptor character (Figure 1c). Meanwhile, complete charge separation is observed in bulk heterojunction (BHJ) films where the polymer is the electron donor (D) and fullerene derivatives are the electron acceptor (A) (Figure 1c).6 Although the initial design ideas for these CTPs were focused on adding quinoidal character to the polymer, which induces a charge-transfer (CT) band to enhance the light harvesting in the red and near-IR regions of the solar spectrum, the CT character in CTPs has brought profound effects in the exciton (EX) dissociation, charge separation, and charge migration steps in the bulk heterojuction (BHJ) devices with an active layer composed of these D−A pairs (Figure 1b).7−9 One family © XXXX American Chemical Society
Received: March 26, 2014 Accepted: May 13, 2014
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dx.doi.org/10.1021/jz5005957 | J. Phys. Chem. Lett. 2014, 5, 1856−1863
The Journal of Physical Chemistry Letters
Letter
Figure 1. (a) PTBF polymers. (b) Possible intermediate states and their energy levels in BHJ films. (See the text.) (c) Illustration of -d-a- motifs and donor (D) polymer and fullerene acceptor (A).
defined by Δμeg = μe − μg (where μe is the excited-state dipole and μg is the ground-state dipole of the polymer).6,7,15,16 Although this value could be diverse due to the different isomers produced via the rotation along certain C−C bonds along the polymer chain,18 its averaged value should reflect the ensemble in a series of CTPs with identical backbones such as the PTBF polymers (Figure 1). This figure of merit was justified in our previous study on the PTBF polymers in solution, which showed that a higher Δμeg gives a higher initial population ratio of the polymer cation, representing the CS state, to the bound pairs representing the CT state.7 Here we report our studies of these systematically altered PTBF polymers in BHJ films, focusing on the role of the local exciton polarity on the exciton dissociation dynamics at the DA interface between CTPs and PCBM using near-infrared (NIR) transient absorption (TA) spectroscopy.7 In particular, we focus on the importance of local structure in -d-a- motifs of these PTBF CTPs on exciton dissociation dynamics over a time scale of approximately 100 fs to 100 ps in BHJ films as well as the corresponding device properties. The results on these time scales elucidate the characteristics of the following mechanisms in BHJ films: (1) the degree of electron transfer in the CTP BHJ films, (2) the dissociation of the cationic species from its anionic counterpart, (3) the dependence of the local electronic structure on the Coulombic interaction between charges in the D and A species, and (4) the excitation energy dependence on the dissociation time of the exciton. Such studies will provide guidance in designing high-efficiency BHJ materials for highperformance OPV devices. The general exciton dynamics and photophysical pathways are illustrated in Figure 1b. The most important three transient species, EX (exciton), CT (charge transfer), and CS (cation), all have absorption in the NIR region.7 The NIR TA spectra for each of the PTBF polymers in BHJ films exhibit a broad absorption signal across the 900−1400 nm window at early delay times (i.e., PTBF2 > PTBF3, suggesting that the cations are stabilized most in PTBF3 and least in PTBF0. This trend, which only occurs when the polymer is in the presence of PCBM, can be understood as an effect of the electron density depletion in the backbone of the polymers. The fluorine atoms draw electron density toward the periphery
and lower the net electron density on the backbone, which is equivalent to increasing the positive charge from the perspective of a nearby PCBM anion and therefore enhances the electrostatic interaction between the D-A pair, localizing the CS state. PTBF3 has the lowest electron density on the backbone, so it has the lowest energy for the cation or CS state. The stronger Coulombic attraction between the anionic PCBM and cationic PTBF near the PTBF-PCBM interface adequately explains the initial blue shift of the CS (cation) peak in the PTBF film spectra compared with those in solution, where the intramolecular charge separation is screened by the surrounding solvent molecules. When comparing the cation peak energies for all four PTBF polymers in solution and in BHJ films, we observed the dependence of the cation dynamics in the two media in terms of (1) the initial cation peak energy, (2) the peak position shift; (3) the local structure-dependent kinetics for the peak shift, and (4) the excitation energy dependence of the cation dynamics. As previously discussed, the cation peak positions for all four polymers in solution are unchanged from 100 fs to a few nanoseconds after excitation,7 but they shift to longer wavelengths continuously in the corresponding BHJ films on the time scale of
